1. Field of the Invention
The present invention relates to sensors.
To be more precise, the present invention relates to a measuring device using an indirect measurement of permittivity between two electrically conductive bodies respectively forming a measuring probe and a reference probe.
2. Description of Related Art
There are many sensors based on measuring permittivity or of the capacitive type.
In particular, there are many devices in which a measuring capacitor is connected to an oscillator circuit so that the output frequency of that circuit depends on the capacitance of the measuring capacitor, enabling determination of a parameter influencing the permittivity of the capacitor, for example the level of a liquid contained in a tank in which the measuring capacitor is placed (see, for example, documents WO-A-98/02718, DE-A-4312432 and DE-A-4434338).
Various devices have also been proposed which comprise a measuring capacitor connected to the input of an integrator stage (see, for example, DE-A-3413849 and Journal of Physics E. Scientific Instruments, vol. 22, no 2, 1989). However, these devices have not proved satisfactory until now and for this reason have not been used industrially.
The documents FR-A-2205669, FR-A-2605731, FR-A-2447555, FR-A-2737297, EP-A-0378304 and EP-A-0644432 describe various devices for measuring the time to charge or discharge a measuring capacitor influenced by the parameter to be detected.
Other capacitive measuring devices are described in the documents FR-A-2763124, FR-A-1152556 and U.S. Pat. No. 3,706,980.
The object. of the present invention is to propose new detector means of very high sensitivity.
Another object of the present invention is to propose detector means suited to many applications.
The above targets are achieved in the context of the present invention by a measuring device employing indirect measurement of permittivity and characterized in that it includes two electrically conductive bodies respectively constituting a measuring probe and a reference probe, electrical power supply means adapted to deliver a DC electrical voltage of controlled amplitude, an integrator stage including a capacitor switching system and control means adapted to define a cyclic series of two sequences at a controlled frequency, namely a first sequence during which the electrical power supply means are connected to the measuring probe to apply an electric field between the measuring probe and the reference probe and accumulate electrical charge on the measuring probe and a second sequence during which the electrical power supply means are disconnected from the measuring probe which is connected to a summation point of the integrator stage to transfer its charge into the integrator stage and obtain at the output thereof a signal representative of the permittivity between the measuring probe and the reference probe.
According to another and advantageous feature of the present invention the control means are adapted to apply a stepped voltage to the measuring probe.
According to another and advantageous feature of the present invention the integrator stage includes an operational amplifier, a first integrator capacitor of high capacitance constituting a feedback capacitor of said amplifier and a second capacitor switched between the output and the input of the operational amplifier at the timing rate of the sequences controlled by the control means.
According to another and advantageous feature of the present invention the device includes means for applying a null average voltage to the measuring probe.
According to another and advantageous feature of the present invention the operational amplifier receives on a second input opposite that adapted to be connected sequentially to the measuring probe a voltage of opposite sign to the voltage applied by the power supply means to the measuring probe.
According to another and advantageous feature of the present invention the amplitude of the voltage applied to the second input of the operational amplifier is equal to p.E where E is the amplitude of the voltage applied to the measuring probe during the time T1 and p is the duty factor between the two sequences T1 and T2, i.e. T1=p.T2.